The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

Mechanical Ventilation in Status Asthmaticus

Sue A. Ravenscraft, MD Wednesday, November 6, 1996

The severity of asthma appears to be increasing. Contributing factor may include an over- reliance on inhaled Beta-adrenergic agents, under use of anti-inflammatory agents, environmental pollution, inadequate assessment of worsening airflow obstruction by patient and physician, and lack of access to medical care. In a severe asthma attack no set arterial blood gas value is an absolute indication for intubation. With aggressive treatment, even patients with CO_{2} retention may avoid intubation. Patients require intubation for clinical signs: decreased level of consciousness, respiratory fatigue, and respiratory arrest. Intubation frequently worsens bronchospasm due to stimulation of irritant receptors in the trachea. It is imperative to rapidly take control over the pattern of breathing once endotracheal intubation is accomplished and positive pressure ventilation begun. This always requires sedation, and often paralysis. The combination of instituting positive pressure ventilation and the use of sedatives may lead to hypotension. Until the patient is placed on a mechanical ventilator the patient should be ventilated by hand at slow frequencies (i.e. 10 breaths/min) to avoid excessive gas-trapping. If hypotension develops a fluid bolus should be given as well as a brief period of apnea (30-40 seconds). In order to minimize barotrauma, a strategy of controlled hypoventilation and permissive hypercapnia is frequently employed until bronchospasm improves. Due to high airways resistance and variable inspiratory flow rates, the peak airway pressure will overestimate the true alveolar pressure. In very severe asthma the measured auto-PEEP may under-represent the degree of gas-trapping and the static airway pressure (plateau) may be the best pressure to monitor and should be followed closely. Medications given to treat bronchospasm in an asthmatic requiring mechanical ventilation do not differ significantly from the aggressive treatment of a non-intubated asthmatic. Acute myopathy may develop with the combined use of steroids and neuromuscular blocking agents. Therefore, it is preferable to use heavy sedation and minimize the long term use of paralytic agents. Lactic acidosis (metabolic acidosis) can be a complication of aggressive Beta-agonist therapy and can complicate the respiratory acidosis that is commonly present in these patients. Generally, the faster the onset of the asthma attack (i.e. asphyxial asthma) the shorter the required duration of mechanical ventilation.

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